The present invention relates to color proofing films, or more particularly to a negative working peel-apart photosensitive element capable of producing negative images upon treatment with actinic radiation and peel development.
In the graphic arts, it is desirable to produce a three or more color proof to assist in correcting a set of color separation films prior to using them to produce printing plates. The proof should reproduce the color quality that will be obtained during the printing process. The proof must be a consistent duplicate of the desired halftone image. Visual examination of a color proof should show the color rendition to be expected from press printing using the color separations and any defects on the separations which might need to be altered before making the printing plates.
Color proofing sheets for multicolored printing can be made by using a printing press or proof press. This requires that all of the actual printing steps be performed. Therefore, this conventional method of color proofing is costly and time consuming.
Photoimaging processes can also be used to produce a color proof. There are two general types of photoimaging methods, namely the overlay type and the single sheet type. In the overlay type of color proofing method, an independent transparent plastic support is used for producing an image of each color separation film by applying a photosensitive solution of the corresponding color. A plurality of such supports carrying images of the corresponding colors are then superimposed upon each other over a white sheet to produce a color proofing composite. The primary advantage of the overlay method is that proofs can be made quickly and can serve as a progressive proof by combining any two or three colors in register. However, this type of color proofing method has the disadvantage that the superimposed plastic supports tend to darken the color proofing sheet. As a result, the impression of the color proofing composite thus prepared is vastly different from that of copies actually obtained with conventional printing presses and with proof presses. Examples of such overlay approaches are contained in U.S. Pat. Nos. 3,136,637; 3,211,553; and 3,326,682.
In the single sheet type of color proofing method, a color proofing sheet is prepared by successively producing images of different colors from different color separation films on a single receiver sheet. This can be accomplished by sequentially applying colorants or colored, photosensitive layers to a single opaque support. This method more closely resembles the actual printing process and eliminates the color distortion inherent in the overlay system. Examples of such single sheet approaches are contained in U.S. Pat. Nos. 3,574,049; 3,671,236; 4,260,673; 4,366,223; 4,650,738; 4,656,114; and 4,659,642.
Various processes for producing single sheet color proofs of an image embodying thermal transfer and photopolymerization techniques are known, for example, from U.S. Pat. Nos. 3,060,023; 3,060,024; 3,060,025; 3,481,736; and 3,607,264. In these processes, a photopolymerizable layer coated on a suitable support is imagewise exposed through a color separation film. The surface of the exposed layer is then pressed into contact with an image receptive surface of a separate element. At least one of the elements is heated to a temperature above the transfer temperature of the unexposed portions of the layer. The two elements are then separated, whereby the thermally transferrable, unexposed, image areas of the composite transfer to the image receptive element. If the element is not precolored, the tacky unexposed image may now be selectively colored with a desired toner. The colored matter preferentially adheres to the clear unpolymerized material.
In U.S. Pat. No. 3,721,557, a method for transferring colored images is shown which provides a stripping layer coated between a photosensitive element and a support. When the photosensitive layer is exposed to actinic light and developed, the more soluble portions are selectively removed to produce a visible image. The image carrying support is pressed against an adhesive coated receptor member and the carrier support is subsequently stripped to accomplish the transfer of the image. A fresh layer of adhesive must be applied to the receptor for each subsequent transfer.
U.S. Pat. No. 4,596,757 provides a method for transferring images or solid colors which are subsequently imaged. The photosensitive material comprises a carrier support having sequentially disposed thereon a release layer; a colored, photopolymerizable layer; and an adhesive layer. The material can undergo exposure, lamination to a temporary support, wet development, and then lamination to a receptor sheet. Alternatively, the photosensitive material can be laminated to a receptor sheet, undergo exposure, and then wet processed. Both processes require development in an aqueous medium.
In U.S. Pat. No. 4,489,154, a process is claimed which produces a single layer color proof without wet development. The photosensitive material comprises a strippable cover sheet; a colored photoadherent layer; a nonphotosensitive organic contiguous layer; and a sheet support. The material is exposed and peel developed. The positive or negative image is transferred to a receiver base. A fresh layer of adhesive must be applied to the receptor for each subsequent transfer.
Finally, U.S. Pat. No. 5,300,399 teaches a negative-acting color proofing element. The element sequentially comprises a strippable cover sheet which is transparent to actinic radiation; a crosslinked layer containing a polymer having phenolic groups; a color layer, containing a colorant, a polymeric binder, a polymerizable monomer and, optionally, a photoinitiator; a photoadhering layer, containing a linear photosensitive polymer containing ethylenically unsaturated, free-radical polymerizable groups and having a molecular weight greater than 3,000, a polymerizable monomer, and, optionally, a free radical photoinitiator; a thermoplastic adhesive layer; and a receiver sheet, at least one of the color layer and the photoadhering layer containing the photoinitiator. A single sheet, negative working color proofing film having improved image quality with very high resolution has been found by using the photosensitive polymer in the photoadhering layer of the article of this invention. However, it is sometimes desirable to blanket-expose the finished proof to actinic radiation in order to photoharden the top layer in order to provide blocking-and-scratch resistance. It was found that the photoadhering layer taught in U.S. Pat. No. 5,300,399 becomes discolored (yellows) when exposed to UV-light.
It is an object of the present invention to remedy that unwanted discoloration when the color proofing element is exposed to UV-light.